22 Prof. Sir (J. A. Cameron and Mr. J. Macallan. [May 2, 



' Inorganic Chemistry,' state that it crystallises in six-sided prisms ; 

 bnt in several chemical works the crystals are described as rhombic 

 prisms, while Pelouze and Fremy mention that it forms large trans- 

 parent crystals which are rhomboidal prisms. The crystalline form 

 is thns referred to three different systems. The first description is 

 probably copied by mistake from that of the anhydrous acid. The 

 most recent investigators of the point are Jacquelain ('' Ann ales de 

 Chimie,' vol. 30, 1850, p. 343), and Pierre and Puchot (' Annales de 

 Chimie,' vol. 2, 1874, p. 164). The latter say that the crystalline 

 form appeared to them to be the obliqne rhomboidal prism, and that 

 they obtained the crystals, some very large, others thin and very 

 long. Jacquelain describes them as being obliqne prisms very in- 

 clined and very large ; and states that he obtained them, by a rather- 

 slow crystallisation, distinctly obliqne and very short, and by a qnick 

 crystallisation, in very long obliqne prisms. 



The Existence of higher Hydrates. 



A portion of dilute selenic acid was concentrated on the water-bath 

 and its strength taken : 0*8603 gram neutralised 19*48 c.c. of semi- 

 normal soda solntion, equivalent to 81 '92 per cent, of selenic acid. 

 The acid so prepared was dilnted with sufficient water to reduce its 

 strength to 80"11 per cent., corresponding to a dihydrated acid, 

 H 2 Se0 4 ,2.H 2 0, and its temperature then gradually lowered. When 

 kept at — 51° for some time it became as viscous as thick syrup, but 

 did not freeze. The last acid was then diluted to a strength of 

 57*32 per cent, of selenic acid, corresponding to a hydrate of the 

 composition H 3 Se0 4 ,6H 2 0. The acid thus prepared did not freeze 

 when kept at —49°, and was quite liquid at that temperature. 



Although no proof of the existence of higher hydrates than the 

 monohydrated acid was obtained in the foregoing experiments, it 

 appears probable that a dihydrated acid, and perhaps other hydrates, 

 are capable of existing. Sulphuric and telluric acid have both been 

 obtained as dihydrates. Considerable heat is evolved when mono- 

 hydrated selenic acid is mixed with sufficient water to reduce its 

 strength to that of a dihydrated acid. When the latter is further 

 diluted, there is an additional slight evolution of heat. It is probable 

 that as the freezing point of monohydrated selenic acid is consider- 

 ably below that of the anhydrous acid, so the freezing point of a 

 dihydrated acid is still lower. In order to get an approximate idea 

 of the amount of water which anhydrous seienic acid absorbs, a 

 portion weighing 0*9776 gram was placed on a watch-glass protected 

 from dust, but with free access of air. After twenty-four hours the 

 acid weighed 2'0284 grams, showing an absorption in that time of 

 between 8 and 9 molecules of water by 1 molecule of anhydrous acid. 

 Another portion weighing 0*4416 gram was exposed until it ceased to: 



